Quick-set concrete is a dry concrete mix that has calcium chloride added. Calcium chloride causes the concrete to harden, or set, within 30 minutes or less of adding water to the mix. It can then take up to 4 days to cure and harden to its maximum strength. Keeping quick-set concrete completely dry right up to the point of use is crucial to the successful mixing of the concrete, so it is not advisable to use it in the rain.
While quick-set concrete is still in the bag, moisture can leak into it and cause it to clump. Once it has clumped, it is ruined, as it cannot then be mixed correctly or thoroughly. It is important to stack it on pallets off the floor or ground whether indoors or out. If outdoors, the concrete bags must be stacked and tarped in such a way that water runs off and does not contact the bags, much like a roof is sloped to allow and direct the flow of water run-off.
The proportions of mixing quick-set concrete must be followed carefully in order to get the right slump. Slump is the term used to describe the consistency of the mix, and is determined by piling the mix up and measuring the height of the pile after it slumps, or settles. If mixed in the rain, it is difficult to determine how much rain is getting into the container while mixing; this can unbalance the proportions of materials, causing the concrete to weaken or not mix properly.
It is possible to pour small projects in the rain if your work area is protected from direct rain, and especially if you have a dry shelter adjacent to the project in which to handle and mix the concrete. If setting a post or footing or other small project, it may be possible to set up a quick shelter out of stakes and tarps to shelter the area. Winds, however, can make even this impossible, and a wet pouring area can negatively affect the curing process. Keep in mind the water held by your own clothing, as well, as you handle dry bags of quick-set concrete.
It is also possible to use quick-set concrete without mixing it, relying instead on the already-present moisture in the ground to sufficiently moisten the mix. This works for setting posts. Dig the hole to three times the diameter of the post. Moisten the hole with a garden hose, if it's not raining. Set and level the post using a level or plumb line, and pour the dry quick-set concrete around the post. Avoid standing water in the bottom of the hole -- too much water can cause the cement to weaken. Brace the post so it does not move while the concrete sets up. While the rain will provide plenty of ground moisture, doing this project in the rain may introduce too much groundwater to the mix, weakening the finished product. The rain can also cause problems with the curing process of the concrete.